Armor Class
From Legends of Cphera
Armor Class (AC) is a numerical representation of how hard a character is to hit with an attack.
In order to land a blow, the attacker's attack roll must be equal to or higher than the defender's Armor Class
Also sometimes called Defense.
Armor Class is determined as:
AC: 10 + Dexterity + BDB + Facing + Armor Bonus + Shield Bonus (sometimes) + Miscellaneous Bonus